The present invention relates to a microcapsule (or microencapsulated) product enclosing a core material of a hydrophobic material which is solid or liquid at room temperature, and a (continuous) process for microencapsulation for production thereof. More specifically, the present invention relates to a microcapsule of a hydrophobic material having a stable (laminar) coating structure, and a (continuous) process for production thereof.
Microencapsulation technique is widely adopted for the purpose of, e.g., protection of or controlling the rate of liberation to outside of a comminuted core material or content.
For example, as microencapsulation processes for agricultural chemicals, there have been proposed processes using, e.g., gelatin which is a water-soluble polymer (e.g., Japanese Laid-Open Patent Application (JP-A) 50-99969), polyamide, polyurethane or polyester (JP-A 54-135671), polyvinyl acetate or polyvinyl ether (JP-A 55-92136), polyurethane-polyurea (JP-A 54-91591), and polyamide-polyurea (JP-A 48-4643), respectively, as coating film materials. However, a microcapsule using gelatin as the film material is poor in controllability of persistent chemical effect due to the fact that the film in a dry state becomes too tight to allow liberation of the content and the film in a wet state is swollen to liberate most of the content in a short time. Further, a microcapsule obtained by once-forming a film of a water-soluble polymer such as gelatin and making the film tighter by reacting the film with, e.g., an aminoplast resin prepolymer (JP-A 52-38097) cannot be free from the drawback of liberating the content in a short time in a wet state. Microcapsules comprising film materials of polyurea, polyamide, polyurethane, etc., are produced by interfacial polymerization, for which one of the monomers for constituting the film polymer has to be soluble in the core material and which is therefore not applicable to a core material not having an ability of dissolving the monomer. Further, the interfacial polymerization has drawbacks that some portion of the monomer can remain unreacted to adversely affect the core material capable of dissolving the monomer and the effect of the core material is reduced when the core material is reactive with the monomer.
Other microencapsulation processes include a process of using urea-formamide polycondensate alone (Japanese Patent Publication (JP-B) 46-30282); and a process of dispersing a material to be encapsulated in a dispersion medium in the presence of a reactive tenside, then irreversibly converting the tenside into an insoluble state to form a primary capsule suspension liquid, mixing an aminoplast precondensate solution into the primary suspension liquid and converting the aminoplast precondensate into an insoluble state to form a secondary capsule suspension liquid containing microcapsules provided with a reinforced coating film wall (JP-A 46-7313). However, the latter process using an aminoplast precondensate for forming a film wall is inevitably accompanied with aggregation of the produced microcapsules to result in aggregated particles. As a result, it becomes very difficult to control the rate of liberation of the core material and to recover the microcapsules in an isolated powdery state.
As for a microcapsule comprising a core material uniformly coated with a film material of an amino resin, such as melamine resin, (thio)urea resin or benzoguanamine resin, the applicant company (Kureha Kagaku Kogyo K.K.) has already proposed a process for producing a microcapsule comprising a film material of an amino resin and a water-soluble cationic resin in the presence of an aninoic surfactant (JP-B 2-29642, U.K. Laid-Open Patent Application (GB-A) 2113170). According to the process, polycondensation of an amino resin prepolymer is caused in the co-presence of minor amounts of a water-soluble cationic resin and an anionic surfactant which have mutually opposite polarities of charges, whereby it becomes possible to form a dispersion liquid which is much stabler than in the absence of the latter two materials, thus providing uniform microcapsules.
According to further study of the present inventors, however, the uniformity of the capsule coating layer obtained by the process of the above-mentioned GB-A 2113170 is not necessarily sufficient, and the occurrence of a substantial amount of isolated or aggregated particles of the film material alone not containing the core material can still be recognized together with the occurrence of aggregated microcapsules and uncoated particles of the core material. The isolated or aggregated particles of the film material alone and aggregated microcapsules can be separated from the product microcapsule, but this results in a lower yield of the product along with the presence of isolated core material, and the insufficient uniformity of the coating layer naturally results in a lowering in product performance.
A principal object of the present invention is to provide a microcapsule having a more uniform coating layer on a hydrophobic core material, and also a process for effectively producing such a uniform microcapsule with extreme suppression of the occurrence of isolated or aggregated film material alone, aggregated microcapsules and isolated core material.
According to our further study, for accomplishing the above object, it has been found very effective to use a water-soluble cationic amino resin and an anionic surfactant (used as agents for improving the affinity between an amino resin prepolymer and core material particles in the above-mentioned process of JP-B 2-29642) as agents for forming a coacervate film coating core material particles prior to addition of an amino resin prepolymer to coat the core material particles successively with a solidified layer of the coacervate and a polycondensate of the amino resin prepolymer.
Thus, according to a first aspect of the present invention, there is provided a microcapsule, comprising a particulate core material, and a laminar coating layer including (i) a solidified layer of coacervate of a water-soluble cationic amino resin and an anionic surfactant, and (ii) a layer of polycondensate of amino resin prepolymer, successively coating the particulate core material.
According to a second aspect of the present invention, there is provided a process for producing a microcapsule, comprising:
a first coating step of mixing a water-soluble cationic amino resin and the anionic surfactant in the presence of a hydrophobic core material dispersed in an aqueous medium to coat the dispersed core material with a coacervate of the cationic amino resin and the anionic surfactant, and
a second coating step of adding an amino resin prepolymer into an aqueous dispersion liquid containing the coated dispersed core material and polycondensating the amino resin prepolymer to further coat the coated dispersed core material with a polycondensate of the amino resin prepolymer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.